1. Field of the Invention
This invention relates in general to magnetic recording and, in particular, to playback heads useful with magnetic recorders which record at one recording medium-to-head speed and play back at another.
2. Description Relative to the Prior Art
There are many instances in the art of magnetic recording where a recording medium such as a magnetic tape is transported during recording at a first high speed and, during playback, at a second, much lower, speed. See, for example, "Television and Lunar Exploration", by S. W. Spaulding, Journal of the SMPTE, Vol. 69, January 1960, pg. 39.
As a part of their design, playback heads ordinarily have transducer gaps which are extremely shallow. Attendantly, it is highly desirable that a playback head not abrade against a recording tape while a separate record head effects high speed recording of the tape. Aside from causing the playback transducer gap to vanish gradually, high speed playback head-to-tape contact causes the radius of curvature of the tape contacting surface of the head to increase as well, thereby further accelerating the consumption of the playback transducer gap. (By contrast, record heads have deep gaps and, as a result, are not as susceptible to gap-related problems. Besides, as is known, head efficiency in a record head increases with a decreasing gap depth.)
Certainly, the straight-forward solution to the problem of playback head wear in a recorder employing high speed recording and low speed playback is to provide articulation of the playback head: while high speed recording with a record head, the playback head is moved out of contact with the tape, thereby to prevent the playback head from being worn out; during low speed playback, however, the playback head is moved back into contact with the tape for playback purposes. To articulate the playback head requires fairly precise linkages since such linkages must set the playback head against the recorded tape track, or tracks, during playback. Precision parts mean added expense and, naturally, are to be avoided where possible.
It is well known that the signal loss in decibels during playback of a recorded tape is approximately equal to 55 (h/.lambda.), where h is the separation between the playback head and the tape, and .lambda. is the wavelength of the recorded signal. Complementing this relationship is the fact that the head-to-tape separation h equals ##EQU1## where r.sub.o =the radius of curvature of the tape contacting surface of the playback head
.mu.=viscosity of air (typically 0.265.times.10.sup.-8 pounds seconds per square inch) PA1 u=tape speed (inchs/sec.) PA1 T=tape tension (lbs./in.).
These two relationships, which are both discussed in a paper, "Experimental and Theoretical Investigation of Head to Tape Separation in Magnetic Recordings", IEEE Transactions on Magnetics, Vol. MAG-9, No. 4, December 1973, pages 683-688, are employed to advantage in providing, in a recorder of the type having differing record and playback speeds, an inarticulated wear-free playback head.
The above noted IEEE paper, as well as the following art, may be of interest as background for the present invention: U.S. Pat. No. 3,323,116; U.S. Pat. No. 3,525,987; U.S. Pat. No. 3,678,482; and U.S. Pat. No. 3,821,813.